1. Field of the Invention
The present invention relates to an optical element and an optical system including the optical element, which are suitable for optical apparatuses such as a silver-halide film camera, a video camera, a digital still camera, a projector, a telescope, and an image reading apparatus.
2. Description of the Related Art
Up to now, there has been known an optical system, in which glass having extraordinary partial dispersion and low dispersion, for example, fluorite or FK01 (product name), is used for a positive lens, and high-dispersion glass is used for a negative lens, to thereby correct chromatic aberrations. There has also been known an optical system, in which a fine particle dispersion material obtained by mixing fine particles, such as ITO or TiO2, into a resin material or a resin material having an extraordinary partial dispersion characteristic is used to correct chromatic aberrations (Japanese Patent Application Laid-Open No. 2005-181392 and Japanese Patent Application Laid-Open No. 2006-145823). There has also been known an optical system, in which a diffraction optical element having a negative dispersion characteristic (νd=−3.453) or strong extraordinary dispersion (θgF=0.296) different from normal glass material is used so as to correct chromatic aberrations. There has also been known an optical system in which a periodical structure of a diffraction grating is changed to obtain aspherical efficiency (Japanese Patent Application Laid-Open No. 2000-258685).
When one of the fine particle dispersion material and the resin material each having the extraordinary partial dispersion characteristic is used, the correction of chromatic aberrations of an optical system and a reduction in size of the entire system are facilitated. However, the thickness of a material to be used, of the materials is limited in view of formability. When a material having large absorption and scattering in a visible wavelength region is used as the fine particle dispersion material, it is necessary to minimize the thickness of the material in view of transmittance. In this case, it is important to balance between the correction of chromatic aberrations of the optical system and the reduction in size of the entire system, and the thickness of the material. When a diffraction optical element is used for the optical system, the correction of chromatic aberrations is facilitated. However, when there is diffraction light of unnecessary diffraction order other than image pickup light (design order), the diffraction light acts as flare light and significantly degrades imaging performance. For example, when there is intense light, for example, sunlight outside a screen and the diffraction optical element is exposed to the light, the entire screen becomes blurred (flare or ghost), to thereby reduce the contrast of the entire screen.